0:00:10 > 0:00:14The natural world is beautiful, but complex.
0:00:18 > 0:00:20The skies dance with colour.
0:00:20 > 0:00:22Yaaaay-ya!
0:00:26 > 0:00:31Shapes of great geometrical beauty form and disappear.
0:00:36 > 0:00:40And the planet itself is constantly transformed.
0:00:44 > 0:00:46But this seemingly infinite complexity...
0:00:49 > 0:00:51..is just a shadow of something deeper.
0:00:53 > 0:00:55The underlying laws of nature.
0:00:57 > 0:01:00The world we live in is beautiful to look at,
0:01:00 > 0:01:03but it's even more beautiful to understand.
0:01:21 > 0:01:23Light is our window on the universe.
0:01:25 > 0:01:29By understanding how light is emitted by the sun,
0:01:29 > 0:01:32and how it interacts with the oceans, atmosphere,
0:01:32 > 0:01:35and life on our planet,
0:01:35 > 0:01:38we can explore worlds beyond our solar system...
0:01:41 > 0:01:44..and even search for the telltale signatures of life
0:01:44 > 0:01:46amongst the stars.
0:02:13 > 0:02:16Night on our planet seems eerie,
0:02:16 > 0:02:18other,
0:02:18 > 0:02:20as if all the colour has drained away.
0:02:23 > 0:02:25But in a few places on Earth,
0:02:25 > 0:02:28on just a handful of nights in the year,
0:02:28 > 0:02:30colour bursts through.
0:02:34 > 0:02:3710,000 years ago, at the end of the last ice age,
0:02:37 > 0:02:39the ice sheets melted
0:02:39 > 0:02:44and this part of Iceland rose up and drove the coastline back.
0:02:44 > 0:02:49That left the Skoga River to tumble over the old sea cliffs
0:02:49 > 0:02:51to form that -
0:02:51 > 0:02:54Skogafoss, one of Iceland's great waterfalls.
0:03:01 > 0:03:04When the sun, moon and Earth align,
0:03:04 > 0:03:08moonlight interacts with the spray at the foot of the falls...
0:03:11 > 0:03:13..to form a moonbow.
0:03:18 > 0:03:20Light leaves the sun,
0:03:20 > 0:03:24travels 93 million miles across space,
0:03:24 > 0:03:27and reflects off the surface of the moon.
0:03:28 > 0:03:30And it enters the Earth's atmosphere,
0:03:30 > 0:03:35bounces in and out of water droplets in the waterfall,
0:03:35 > 0:03:36and enters my eye.
0:03:36 > 0:03:39That sends a signal to my brain,
0:03:39 > 0:03:42and I reconstruct the signal as something beautiful.
0:03:51 > 0:03:54But the moonbow isn't just beautiful.
0:03:56 > 0:03:58It's physics.
0:04:08 > 0:04:12Understanding why something's the colour it is tells you something.
0:04:12 > 0:04:14It tells you about its structure,
0:04:14 > 0:04:18about the processes going on inside, about its history, even.
0:04:18 > 0:04:21And because light travels freely across the universe,
0:04:21 > 0:04:25we can explore distant worlds using light alone.
0:04:26 > 0:04:28We can tell their stories, too.
0:04:34 > 0:04:39The reflected light from the moon has its origin in our star, the sun.
0:04:40 > 0:04:43Stars illuminate the universe.
0:04:43 > 0:04:47They are the source of the light that bathes the planets.
0:04:49 > 0:04:53But the processes by which light is emitted by the stars
0:04:53 > 0:04:54can be explored here on Earth.
0:05:08 > 0:05:12On the 31st August, 2014,
0:05:12 > 0:05:15a vast chasm opened up in central Iceland.
0:05:26 > 0:05:29At its peak, the Bardabunga volcano
0:05:29 > 0:05:34spewed out 350 cubic metres of molten rock every second...
0:05:40 > 0:05:42..producing light so bright...
0:05:48 > 0:05:50..it could be seen from space.
0:06:00 > 0:06:05And just like the sun, lava glows because it's hot.
0:06:09 > 0:06:11Just look at that.
0:06:11 > 0:06:15It's many months, since the eruption but it's steaming away back there.
0:06:15 > 0:06:18We're actually fortunate that it erupted here.
0:06:18 > 0:06:22The helicopter pilot told me that if it'd erupted on the glacier,
0:06:22 > 0:06:26then we would have been plunged into a perpetual nuclear winter
0:06:26 > 0:06:29and civilisation would have been destroyed.
0:06:29 > 0:06:30That's not actually quite right,
0:06:30 > 0:06:33but it would have been significantly worse.
0:06:33 > 0:06:35The amount of ash that went up into the atmosphere
0:06:35 > 0:06:38would have been really considerable.
0:06:38 > 0:06:42You get a sense of the power and violence of the Earth.
0:06:42 > 0:06:45Cos we're blissfully unaware of it, usually,
0:06:45 > 0:06:49but when it breaks through the surface, you see what it can do.
0:07:02 > 0:07:07Today the lava has cooled into matt-black rock,
0:07:07 > 0:07:09but it's still giving off light.
0:07:13 > 0:07:17The thing is, everything's hot. Everything has a temperature.
0:07:17 > 0:07:21But if it's cold, it just emits light that we can't see.
0:07:21 > 0:07:23This camera can see it, though.
0:07:23 > 0:07:26It's called infrared light.
0:07:26 > 0:07:28You see there that this lava
0:07:28 > 0:07:31is glowing brighter than the background.
0:07:31 > 0:07:33Means it's hotter.
0:07:37 > 0:07:40So, although we can't see it,
0:07:40 > 0:07:42everything shines.
0:07:47 > 0:07:50Light fills the universe.
0:07:50 > 0:07:53Radio waves are light, X-rays are light.
0:07:53 > 0:07:57Visible light is simply the part of the spectrum we can see.
0:08:01 > 0:08:04Matter, like lava, or you and me,
0:08:04 > 0:08:08is made up of electrically charged particles.
0:08:08 > 0:08:10Here's one moving along -
0:08:10 > 0:08:12say it's an electron.
0:08:13 > 0:08:18Temperature is a measure of how fast those particles are moving around,
0:08:18 > 0:08:19how fast they're jiggling.
0:08:19 > 0:08:21So, in something hot,
0:08:21 > 0:08:24these particles are always bouncing around
0:08:24 > 0:08:25and changing direction.
0:08:25 > 0:08:29Now, here's a fundamental law of the universe.
0:08:29 > 0:08:32When a charged particle changes direction,
0:08:32 > 0:08:35it emits a light particle
0:08:35 > 0:08:37called a photon.
0:08:41 > 0:08:44Light can be thought of as a stream of photons -
0:08:44 > 0:08:47particles whose energy corresponds to their colour.
0:08:54 > 0:08:57Cooler things, like solid black lava,
0:08:57 > 0:09:02emit lower energy photons - infrared light, which our eyes can't detect.
0:09:08 > 0:09:12As things get hotter, they can radiate higher-energy photons.
0:09:12 > 0:09:17At 1,000 degrees Celsius, molten lava shines with mainly red light,
0:09:17 > 0:09:20which our eyes can detect.
0:09:21 > 0:09:26The surface of the sun is 5,500 degrees Celsius,
0:09:26 > 0:09:30so it can also produce higher-energy green and blue photons
0:09:30 > 0:09:31alongside the red.
0:09:34 > 0:09:36The white light of the sun
0:09:36 > 0:09:39is made up of all the colours of the rainbow.
0:09:41 > 0:09:43And when it reaches the Earth,
0:09:43 > 0:09:46those individual colours are revealed.
0:09:55 > 0:09:59Photons from the sun rain down and enter water droplets.
0:09:59 > 0:10:02They reflect off the rear face,
0:10:02 > 0:10:05come out of the front again and into your eye.
0:10:05 > 0:10:09But the blue photons, the higher-energy ones,
0:10:09 > 0:10:11behave differently to the green ones
0:10:11 > 0:10:13and the yellow ones and the red ones.
0:10:13 > 0:10:15They come out at a shallower angle,
0:10:15 > 0:10:19and that's why you get the full spectrum of colours
0:10:19 > 0:10:21from the white light of the sun.
0:10:24 > 0:10:26And it's this light that paints the Earth.
0:10:35 > 0:10:38The colours of our planet arise because of the way photons
0:10:38 > 0:10:42from the sun interact with the matter from which the Earth is made.
0:10:53 > 0:10:57From space, our planet is a blue world.
0:10:59 > 0:11:00And that blue colour arises
0:11:00 > 0:11:03because of the way light interacts with water.
0:11:06 > 0:11:10And the process is linked to one of the planet's greatest migrations.
0:11:25 > 0:11:29Every spring, marine biologist Osvaldo Vasquez
0:11:29 > 0:11:30heads out onto the high seas...
0:11:32 > 0:11:34- MAN ON RADIO:- ...at the port bow, over.
0:11:34 > 0:11:3511, 11 o'clock.
0:11:37 > 0:11:41..in search of these waters' most awe-inspiring seasonal visitors.
0:11:53 > 0:11:54Humpback whales.
0:12:01 > 0:12:06Every year, they make the longest migration of any mammal on Earth,
0:12:06 > 0:12:08travelling up to 8,000km
0:12:08 > 0:12:12from their feeding grounds in the northern Atlantic
0:12:12 > 0:12:16to the striking blue waters of the Silver Bank Marine Reserve.
0:12:19 > 0:12:25This is the greatest nursery of humpback whales in the world.
0:12:25 > 0:12:3185% of the North Atlantic population comes here for breeding,
0:12:31 > 0:12:33mating, and giving birth.
0:12:35 > 0:12:40So, from this place depends the survival of the species.
0:12:43 > 0:12:46The whales' life cycle is intricately linked
0:12:46 > 0:12:49to the interaction between the light of the sun
0:12:49 > 0:12:51and the water of the ocean.
0:12:55 > 0:12:58In order to understand their behaviour,
0:12:58 > 0:13:00we need to go beneath, and to see,
0:13:00 > 0:13:03with our own eyes, what is happening there.
0:13:05 > 0:13:07Not just on the surface - underwater.
0:13:32 > 0:13:34Yep, yep, yep.
0:13:55 > 0:13:58Slow down. Slow down.
0:13:58 > 0:14:00Slow down, stay down.
0:14:09 > 0:14:11WHALES SING SOFTLY
0:14:17 > 0:14:20The reason humpbacks come here to give birth
0:14:20 > 0:14:25is because the shallow waters around Silver Bank are exceptionally warm.
0:14:29 > 0:14:32Whales are warm-blooded animals,
0:14:32 > 0:14:35and when they give birth,
0:14:35 > 0:14:39they have a very skinny calf with no blubber.
0:14:41 > 0:14:44They need a warm environment, as they had inside Mother.
0:14:45 > 0:14:49So here, Silver Bank, is warm and is protected.
0:14:49 > 0:14:51So it's suitable for giving birth.
0:14:54 > 0:14:57As photons of light rain down onto the ocean,
0:14:57 > 0:15:00they strike water molecules,
0:15:00 > 0:15:03and some of this energy goes into making them jiggle around,
0:15:03 > 0:15:08increasing their speed and, therefore, their temperature.
0:15:10 > 0:15:14But it takes an awful lot of energy to heat the oceans,
0:15:14 > 0:15:16and so it's only here,
0:15:16 > 0:15:19exposed to the full glare of the equatorial sun,
0:15:19 > 0:15:21that enough energy is absorbed
0:15:21 > 0:15:26to raise the water temperature to a balmy 26 degrees.
0:15:26 > 0:15:31The perfect conditions for mothers to raise their calves.
0:15:46 > 0:15:48The calves are really very cute
0:15:48 > 0:15:51because they are like puppies.
0:15:52 > 0:15:56But underwater, it weighs one tonne. So...it's a big puppy.
0:16:01 > 0:16:05But not all of the photons that enter the water are absorbed.
0:16:05 > 0:16:08Some of them bounce straight back out again,
0:16:08 > 0:16:11and it's these photons that reach our eyes
0:16:11 > 0:16:14and create the blue of the oceans.
0:16:21 > 0:16:24To understand why it's mainly red photons that are absorbed
0:16:24 > 0:16:26rather than the blue ones,
0:16:26 > 0:16:30we need to take a closer look at the structure of water.
0:17:07 > 0:17:09This is called Thingvellir.
0:17:11 > 0:17:14That's a rough pronunciation.
0:17:14 > 0:17:17But in Icelandic, it means "parliament valley",
0:17:17 > 0:17:21because the Vikings used to meet here over a thousand years ago
0:17:21 > 0:17:23and had the world's first parliament.
0:17:23 > 0:17:25The thing is, it's not actually a valley,
0:17:25 > 0:17:29in the sense that it wasn't cut by a river or a glacier.
0:17:29 > 0:17:31It's actually formed
0:17:31 > 0:17:34by the continents themselves splitting apart.
0:17:34 > 0:17:37So that is the American continental plate
0:17:37 > 0:17:41and that is the Eurasian continental plate.
0:17:41 > 0:17:44And if you just go about a mile down the road,
0:17:44 > 0:17:47this place is flooded, and you can dive in it.
0:17:53 > 0:17:57The rift is filled with glacial meltwater that seeps down
0:17:57 > 0:17:59from Iceland's frozen interior.
0:18:03 > 0:18:06The sun's rays are much weaker here.
0:18:06 > 0:18:09But, wherever sunlight hits water molecules,
0:18:09 > 0:18:11it always produces the same colour.
0:18:20 > 0:18:21How's that feel?
0:18:23 > 0:18:24That's good.
0:18:40 > 0:18:42Quite nice in here, actually.
0:18:59 > 0:19:00Look how blue this water is.
0:19:01 > 0:19:03You can see for miles.
0:19:03 > 0:19:06I've never seen anything like it.
0:19:06 > 0:19:12The reason for that purity is that the water's come 50km,
0:19:12 > 0:19:16sometimes going deep underground,
0:19:16 > 0:19:19and being filtered by these rocks.
0:19:19 > 0:19:23So it's some of the coldest, purest water you can imagine.
0:19:24 > 0:19:29Really, really spectacular - look at that.
0:19:29 > 0:19:30You can see forever.
0:19:38 > 0:19:40Descending deeper into the fissure,
0:19:40 > 0:19:43the effect on the colour of my drysuit
0:19:43 > 0:19:45hints at the process by which
0:19:45 > 0:19:48the oceans acquire their distinctive blue.
0:19:55 > 0:19:57Well, now we're down at about, erm...
0:19:58 > 0:20:00..well, 12 metres.
0:20:01 > 0:20:06And you see that down here, my beautiful red diving suit
0:20:06 > 0:20:09is no longer quite as red as it was. HE LAUGHS
0:20:10 > 0:20:12In fact, it's looking quite black.
0:20:13 > 0:20:14And the reason for that...
0:20:15 > 0:20:19..is the structure of water molecules themselves.
0:20:21 > 0:20:26See, a water molecule is an oxygen with two hydrogens attached.
0:20:27 > 0:20:32And when the light streams into the water, the red light
0:20:32 > 0:20:37is just the right energy to start those molecules vibrating,
0:20:37 > 0:20:41and they do. They start going like that...
0:20:41 > 0:20:43and like that....
0:20:43 > 0:20:45and even like that.
0:20:45 > 0:20:48So all the energy in the red light is taken,
0:20:48 > 0:20:51making the water molecules wobble and vibrate.
0:20:54 > 0:20:56The blue light doesn't do that.
0:20:56 > 0:20:59So the blue light can scatter around in the water
0:20:59 > 0:21:01relatively unimpeded.
0:21:02 > 0:21:05And that's why water is blue.
0:21:07 > 0:21:08Cool, isn't it?
0:21:19 > 0:21:22Vibrations inside the water molecules themselves
0:21:22 > 0:21:24give the oceans their colour.
0:21:25 > 0:21:29The weaker bonds between water molecules also absorb sunlight.
0:21:31 > 0:21:34Together, these processes absorb so much energy
0:21:34 > 0:21:38that there isn't a lot left to increase the sea temperature.
0:21:38 > 0:21:40WHALE SQUEAKS
0:21:40 > 0:21:43Which is why humpbacks must travel so far
0:21:43 > 0:21:45to find water warm enough to give birth.
0:21:55 > 0:21:57WHALE SINGS
0:22:05 > 0:22:07Because water is hard to heat up,
0:22:07 > 0:22:11the temperature of the oceans remains very stable,
0:22:11 > 0:22:15creating the perfect conditions for life to thrive.
0:22:26 > 0:22:29Liquid water is the essential ingredient
0:22:29 > 0:22:32for any life-supporting world.
0:22:35 > 0:22:40Oceans cover 70% of the Earth's surface,
0:22:40 > 0:22:44their distinctive blue broadcasting to the universe
0:22:44 > 0:22:48the message that Earth is a possible home for life.
0:22:53 > 0:22:56And beyond the waves, on the land,
0:22:56 > 0:22:59we see life's other signature colour -
0:22:59 > 0:23:01a blanket of green plants.
0:23:03 > 0:23:07As with all living things, they depend on water to survive.
0:23:07 > 0:23:10And when the seasonal rains recede,
0:23:10 > 0:23:14the land reverts to the arid colours of the naked earth.
0:23:26 > 0:23:29The intense African sun
0:23:29 > 0:23:33delivers 1,000 watts of solar power per square metre,
0:23:33 > 0:23:37baking the plains of the Serengeti to dust.
0:23:41 > 0:23:43Scorched brown -
0:23:43 > 0:23:46barely a single blade of green grass survives.
0:23:50 > 0:23:55For the Maasai, the dry season means months of hunger and hardship.
0:24:00 > 0:24:03Parakapuni and his son and brother
0:24:03 > 0:24:05must say goodbye to their home valley...
0:24:07 > 0:24:10..and drive their cows across the plains
0:24:10 > 0:24:13in search of enough grass and water
0:24:13 > 0:24:15to keep them alive until the rains return.
0:24:55 > 0:24:57At the end of their long journeys,
0:24:57 > 0:25:00Maasai warriors from across the Serengeti
0:25:00 > 0:25:03gather on the banks of Lake Masek.
0:25:36 > 0:25:39But survival on the plains of the Serengeti
0:25:39 > 0:25:44forces Parakapuni and his family to spend the dry season apart.
0:25:47 > 0:25:50Because while he tends the cattle,
0:25:50 > 0:25:52his wife Nomipuni must stay behind
0:25:52 > 0:25:57to look after those too young or too old to make the trip...
0:25:58 > 0:26:02..and eke out an existence on the brown, lifeless plains.
0:26:29 > 0:26:30With the cows away,
0:26:30 > 0:26:33the family must survive on their meagre stores
0:26:33 > 0:26:35of maize flour and water.
0:26:38 > 0:26:39YOUNGER CHILD WAILS
0:27:30 > 0:27:31THUNDER RUMBLES
0:27:31 > 0:27:34Now, after four long months apart,
0:27:34 > 0:27:37things are about to change.
0:27:47 > 0:27:49NOMIPUNI:
0:28:07 > 0:28:10The coming of the rains means the grass can grow again.
0:28:19 > 0:28:23Revived, new shoots emerge,
0:28:23 > 0:28:26packed with the green pigment chlorophyll...
0:28:30 > 0:28:34..which allows plants to harvest the sun's energy.
0:28:39 > 0:28:45And the plains are transformed from dusty brown to vibrant green.
0:28:50 > 0:28:54In just a few days, the landscape is reborn.
0:29:15 > 0:29:17I suppose we think of green as the colour of life,
0:29:17 > 0:29:21but actually, it's the colour that life throws away,
0:29:21 > 0:29:25waste green photons reflected back into our eyes.
0:29:25 > 0:29:28Plants absorb most of the rest of the rainbow,
0:29:28 > 0:29:30the blue and red photons,
0:29:30 > 0:29:33and use their energy to power photosynthesis.
0:29:36 > 0:29:42Photosynthesis is the process by which plants harness light.
0:29:42 > 0:29:47They are the bridge between nuclear reactions 93 million miles away
0:29:47 > 0:29:49and life on Earth.
0:29:52 > 0:29:56Energy released from nuclear fusion reactions in the sun's core
0:29:56 > 0:29:59heats everything up and shakes electrons around,
0:29:59 > 0:30:03and those electrons will emit photons,
0:30:03 > 0:30:06which travel across space for eight minutes,
0:30:06 > 0:30:11and then hit an electron in a chlorophyll molecule.
0:30:11 > 0:30:15But instead of that energy being dissipated away as heat,
0:30:15 > 0:30:17chlorophyll is clever,
0:30:17 > 0:30:21and ultimately the energy imparted to that electron
0:30:21 > 0:30:23is used to do all sorts of clever things
0:30:23 > 0:30:26through an intricate piece of machinery.
0:30:26 > 0:30:29Split water up, force electrons onto carbon dioxide,
0:30:29 > 0:30:34and ultimately, build sugars, which allows the plant to grow.
0:30:38 > 0:30:43On the plains of the Serengeti, the intricate process of photosynthesis
0:30:43 > 0:30:48means Parakapuni can begin the long journey to rejoin his family.
0:31:00 > 0:31:04After months apart, the greening of the Serengeti
0:31:04 > 0:31:06transforms the harshest of environments
0:31:06 > 0:31:10into a place the Maasai can call home.
0:32:12 > 0:32:15Animals eat plants that feed on sunlight.
0:32:21 > 0:32:25In this way, the sun's energy powers the entire food chain.
0:32:31 > 0:32:35But photosynthesis does more than provide life with energy.
0:32:39 > 0:32:42Around two and a half billion years ago, it began to transform
0:32:42 > 0:32:46the composition of the Earth's atmosphere itself,
0:32:46 > 0:32:50by filling it with a waste product - oxygen.
0:32:50 > 0:32:54The gas upon which all complex life depends.
0:33:04 > 0:33:08Without air to breathe, there would be no intelligent life on Earth.
0:33:15 > 0:33:18We rely absolutely on the oxygen that forms
0:33:18 > 0:33:22a fifth of the thin blue line that envelops our planet.
0:33:29 > 0:33:32And, perhaps paradoxically,
0:33:32 > 0:33:35why the sky is blue is best explained
0:33:35 > 0:33:37in the dead of night.
0:33:50 > 0:33:53It's a beautiful, crisp autumn night here in the south of England.
0:33:53 > 0:33:56Rolling hills illuminated by moonlight.
0:33:58 > 0:34:00Which is the light from the sun
0:34:00 > 0:34:02reflected off the surface of the moon.
0:34:05 > 0:34:08It's quite silent, actually, almost eerie.
0:34:08 > 0:34:12But out there, the Earth's shadow is racing through space,
0:34:12 > 0:34:15and actually it's just beginning to cut off
0:34:15 > 0:34:18the top left-hand corner of the moon.
0:34:22 > 0:34:24On a few nights every decade,
0:34:24 > 0:34:28the sun, moon and Earth line up
0:34:28 > 0:34:31to create one of the wonders of the night sky.
0:34:31 > 0:34:33A total lunar eclipse.
0:34:36 > 0:34:40You get a real sense of the celestial mechanics in action
0:34:40 > 0:34:42during an eclipse.
0:34:42 > 0:34:45The sun is somewhere over there behind me,
0:34:45 > 0:34:47shining on our planet,
0:34:47 > 0:34:51and our planet is casting a shadow through space,
0:34:51 > 0:34:55which falls on the surface of our satellite, the moon.
0:35:00 > 0:35:03As the Earth moves between the moon and the sun,
0:35:03 > 0:35:07it blocks light from falling directly onto the lunar surface.
0:35:15 > 0:35:18Now the moon is completely covered by the Earth's shadow.
0:35:19 > 0:35:21But it's not completely blank.
0:35:21 > 0:35:24It's glowing a deep blood red.
0:35:28 > 0:35:32At the height of the eclipse, the only photons reaching the moon
0:35:32 > 0:35:35have passed through the atmosphere of our planet.
0:35:39 > 0:35:42And it's red because of the way that light
0:35:42 > 0:35:45interacts with the Earth's atmosphere.
0:35:45 > 0:35:49Imagine the rain photons passing through the atmosphere
0:35:49 > 0:35:52from the sun, all the colours of the rainbow.
0:35:52 > 0:35:56Well, the blue photons, the higher-energy photons,
0:35:56 > 0:36:00have a higher probability of bouncing off the molecules
0:36:00 > 0:36:03in the Earth's atmosphere so they scatter around.
0:36:05 > 0:36:08That just leaves the lower-energy red photons,
0:36:08 > 0:36:12which have a much lower probability of bouncing around,
0:36:12 > 0:36:15and therefore can pass through the Earth's atmosphere
0:36:15 > 0:36:17relatively unscathed.
0:36:17 > 0:36:22So they are the ones that make it onto the moon's surface,
0:36:22 > 0:36:25and reflect back into my eye, and that's why,
0:36:25 > 0:36:28in a lunar eclipse, you get a blood-red moon.
0:36:40 > 0:36:44And by day, the blue photons the atmosphere scatters
0:36:44 > 0:36:46are what paint the skies blue.
0:36:50 > 0:36:52So when you look up into the sky on a summer's day
0:36:52 > 0:36:54and see that deep blue,
0:36:54 > 0:36:58what you're actually seeing is blue photons
0:36:58 > 0:37:01scattering around off the molecules in the atmosphere.
0:37:08 > 0:37:12Red, green, blue -
0:37:12 > 0:37:15the primary colours of our planet.
0:37:15 > 0:37:18The Earth is painted by the photons that rain down
0:37:18 > 0:37:20from the surface of the sun.
0:37:22 > 0:37:28But for us to see them, those photons must take one final journey.
0:37:53 > 0:37:56Salma lives with her seven-year-old son Majidil
0:37:56 > 0:37:59on the banks of the Gorai River in Bangladesh.
0:38:18 > 0:38:21This is a vibrant place to grow up.
0:38:22 > 0:38:27But unlike the other children, Majidil can't experience it.
0:38:49 > 0:38:54For us to see, photons must travel into our eyes,
0:38:54 > 0:38:57but Majidil was born with severe cataracts.
0:38:59 > 0:39:01The lenses of his eyes are so clouded,
0:39:01 > 0:39:05he's been effectively blind since birth.
0:39:26 > 0:39:30But today, Majidil's life is about to change.
0:39:38 > 0:39:42His family are making the eight-hour journey to the capital, Dhaka.
0:39:50 > 0:39:51Because this afternoon,
0:39:51 > 0:39:55Majidil will undergo surgery designed to restore his sight.
0:40:14 > 0:40:16If the surgery is successful,
0:40:16 > 0:40:20replacing his faulty lenses with artificial ones
0:40:20 > 0:40:23will allow light to travel directly into Majidil's eyes.
0:40:36 > 0:40:39It's at the back of the eye that photons collide with cells
0:40:39 > 0:40:43in the retina, sensitive to red, green, and blue light...
0:40:46 > 0:40:50..and trigger the nerve impulses that allow us to see in colour.
0:41:04 > 0:41:08BICYCLE BELL RINGS
0:41:26 > 0:41:29THEY SPEAK ENCOURAGINGLY TO HIM
0:41:40 > 0:41:42For the first time in his life,
0:41:42 > 0:41:45photons are able to enter Majidil's eyes unimpeded.
0:41:53 > 0:41:57For the first time in his life, he can see.
0:42:46 > 0:42:49Our eyes reveal to us the colours of the Earth.
0:42:49 > 0:42:51MAJIDIL LAUGHS
0:42:54 > 0:42:58The first step towards understanding their meaning.
0:43:04 > 0:43:07The blue that signals the presence of liquid water.
0:43:10 > 0:43:15The green that marks out our planet as a home for complex life.
0:43:18 > 0:43:22And, in this most distant image of the Earth ever taken,
0:43:22 > 0:43:25from six billion kilometres away,
0:43:25 > 0:43:27by the Voyager spacecraft,
0:43:27 > 0:43:30the sunlight bouncing off our atmosphere
0:43:30 > 0:43:33reveals our planet as a pale blue dot
0:43:33 > 0:43:36hanging in the void of space.
0:43:38 > 0:43:43And even from this distance, anyone who happened to be looking
0:43:43 > 0:43:48could tell our planet contained the ingredients for life.
0:43:49 > 0:43:52Because in the most delicate colour signals,
0:43:52 > 0:43:56we have measured with precision what the universe is made of.
0:44:02 > 0:44:07And those signals can be glimpsed at the very edge of our own atmosphere.
0:44:22 > 0:44:24What are we drawing?
0:44:24 > 0:44:26A sun dragon.
0:44:26 > 0:44:27What's a sun dragon?
0:44:27 > 0:44:29It's just like a sun.
0:44:30 > 0:44:34And it's got smoke coming out of its mouth.
0:44:36 > 0:44:40Cos it's starting to make fire on the world.
0:44:40 > 0:44:41SHE PRETENDS TO BREATHE FIRE
0:44:42 > 0:44:46Now, let's just do them there.
0:44:46 > 0:44:48We need to get ready to go, don't we?
0:44:50 > 0:44:51Push.
0:44:51 > 0:44:55For seven years, the Nation family have been consumed by an obsession.
0:44:58 > 0:45:01Ready to drop everything at a moment's notice,
0:45:01 > 0:45:04in pursuit of a light show like no other.
0:45:08 > 0:45:10I'm Aurora Chaser!
0:45:15 > 0:45:18They've left their home on the Sussex coast
0:45:18 > 0:45:21for the cold climes of northern Norway...
0:45:21 > 0:45:2373, 98, 97.
0:45:25 > 0:45:2769, 98, 100.
0:45:27 > 0:45:29..spending their weekends
0:45:29 > 0:45:33chasing one of the planet's most extraordinary phenomena -
0:45:33 > 0:45:35the Northern lights.
0:45:35 > 0:45:36Aurora borealis.
0:45:39 > 0:45:43It's an addiction. It's an absolute full-blown addiction.
0:45:43 > 0:45:46It's a buzz and it's a rush of adrenaline.
0:45:46 > 0:45:50And it is a holding of your breath and...wow.
0:45:52 > 0:45:54The aurora was once thought to be caused
0:45:54 > 0:45:56by dragons fighting in the sky.
0:45:59 > 0:46:01But today, we know its ethereal light
0:46:01 > 0:46:05is created by charged particles known as the solar wind
0:46:05 > 0:46:09interacting with the gases of the upper atmosphere.
0:46:14 > 0:46:16Our eldest, Aurora, she's 11.
0:46:16 > 0:46:17She's fascinated by,
0:46:17 > 0:46:18why do those different gases
0:46:18 > 0:46:20make those different colours?
0:46:20 > 0:46:23She's quite fascinated by it. Oce...
0:46:23 > 0:46:25she likes the pink, because she likes pink.
0:46:25 > 0:46:27It's her favourite colour, so pink's cool.
0:46:27 > 0:46:30Anything else, just wake me up if it goes pink.
0:46:30 > 0:46:33And Lyrica is quite a fact box on everything.
0:46:33 > 0:46:36She can explain to you what causes an aurora,
0:46:36 > 0:46:38where it comes from, the technical names,
0:46:38 > 0:46:41and, you know, what the magnetic field does
0:46:41 > 0:46:44and how it stops the solar wind from harming us.
0:46:49 > 0:46:51But the aurora is elusive.
0:46:51 > 0:46:55To see it, conditions have to be just right.
0:46:59 > 0:47:01Here we go!
0:47:01 > 0:47:06Clear dark skies must combine with strong solar winds.
0:47:15 > 0:47:16You're going to burn it.
0:47:28 > 0:47:31All the family can do is wait.
0:47:56 > 0:47:59Oh, oh, oh, oh. I can see some uprights there.
0:48:01 > 0:48:04Oh, wow.
0:48:13 > 0:48:15- Oh, yeah. - Here we go. Look at it.
0:48:17 > 0:48:19Beautiful!
0:48:20 > 0:48:22Look at it!
0:48:23 > 0:48:25Look how bright that is up there, look!
0:48:34 > 0:48:36Yay! Yes!
0:48:37 > 0:48:40Now that's what I'm talking about.
0:48:42 > 0:48:44Look. Ooh!
0:48:47 > 0:48:50Look, look, look, look, look! Corona, corona!
0:48:51 > 0:48:54- Look at it. - Look at the reds at the top!
0:48:54 > 0:48:56- Stunning.- Wow.
0:49:01 > 0:49:02Oh, oh, oh!
0:49:02 > 0:49:05Oh, look at that.
0:49:07 > 0:49:10- And on the left, on the left, look, there's some pink.- Yay!
0:49:10 > 0:49:12Look, look, look, look, look!
0:49:14 > 0:49:17But the aurora isn't simply beautiful.
0:49:17 > 0:49:19It contains information.
0:49:25 > 0:49:29As charged particles strike gas atoms,
0:49:29 > 0:49:30a photon of light is released.
0:49:34 > 0:49:38And because the atoms of every element have a unique structure,
0:49:38 > 0:49:42each one gives off a photon of a unique colour.
0:49:46 > 0:49:49The green comes from oxygen atoms,
0:49:49 > 0:49:51pink from nitrogen.
0:50:00 > 0:50:02Yes!
0:50:05 > 0:50:07The colours dancing across the sky
0:50:07 > 0:50:10reveal the composition of our atmosphere.
0:50:13 > 0:50:17And it's these fingerprints encoded in photons of light
0:50:17 > 0:50:21that have shown us the entire universe.
0:51:16 > 0:51:18In the last few hundred years,
0:51:18 > 0:51:22astronomers have gazed out from the edges of our home world...
0:51:24 > 0:51:27..and dreamt of the universe beyond.
0:51:36 > 0:51:40In the shadow of the Eiger, that is the Sphinx.
0:51:40 > 0:51:41And despite appearances,
0:51:41 > 0:51:45it isn't the lair of some Alpine super-villain.
0:51:45 > 0:51:46It's an observatory.
0:51:46 > 0:51:48It was built in 1937, and since then,
0:51:48 > 0:51:52astronomers have been coming here from all over the world
0:51:52 > 0:51:53to look at the night sky.
0:52:03 > 0:52:09And by decoding messages hidden in the light reaching our telescopes,
0:52:09 > 0:52:12we've discovered the true nature of the cosmos.
0:52:18 > 0:52:20That's an aurora in a box.
0:52:20 > 0:52:24It's a tube full of hydrogen gas that's heated up.
0:52:24 > 0:52:28And what you're seeing in that pinky-red colour
0:52:28 > 0:52:33are the fundamental laws of nature themselves in action.
0:52:33 > 0:52:39They tell you that a single proton, which is the nucleus of hydrogen,
0:52:39 > 0:52:43and a single electron, can only behave in certain ways.
0:52:43 > 0:52:47The electrons can only go in certain places around the proton,
0:52:47 > 0:52:49so when you heat them up,
0:52:49 > 0:52:51they can only jump high up.
0:52:53 > 0:52:54And then when they cool down,
0:52:54 > 0:52:57they can only fall back in a specific way,
0:52:57 > 0:53:00releasing light of a particular energy
0:53:00 > 0:53:02which equals a particular colour.
0:53:02 > 0:53:05And those laws don't just apply here on Earth.
0:53:05 > 0:53:08They apply everywhere across the universe.
0:53:08 > 0:53:13So anywhere that you see glowing hydrogen gas, let's say in a nebula,
0:53:13 > 0:53:18heated up by young stars, then you'll see that specific colour.
0:53:25 > 0:53:28Vast clouds of glowing hydrogen
0:53:28 > 0:53:31create some of the most spectacular sights in the cosmos...
0:53:35 > 0:53:38..their pink hue revealing the identity of the gas
0:53:38 > 0:53:40from which they're made.
0:53:52 > 0:53:55And what's true for nebulae
0:53:55 > 0:53:58is true for everything in the universe.
0:54:01 > 0:54:03By analysing the light from the stars,
0:54:03 > 0:54:07we know that they're not just twinkling lights in the sky.
0:54:07 > 0:54:10They're other suns made out of the same stuff as ours.
0:54:10 > 0:54:14And if there are other suns, then there must be other planets -
0:54:14 > 0:54:16maybe even other Earths.
0:54:32 > 0:54:36By understanding the origin of the colours of our world -
0:54:37 > 0:54:39the blue of its oceans...
0:54:42 > 0:54:43..the green of its life...
0:54:47 > 0:54:50..and the colours dancing in its atmosphere...
0:54:52 > 0:54:54..we've uncovered the colour signature
0:54:54 > 0:54:56of a life-supporting planet.
0:55:01 > 0:55:05And because we understand the signature of our own world,
0:55:05 > 0:55:10we can look for worlds like ours elsewhere in the cosmos.
0:55:17 > 0:55:20Here's what I think is the most wonderful thought of all.
0:55:20 > 0:55:25When you look out to those distant star systems and planets,
0:55:25 > 0:55:27you're connected to them.
0:55:27 > 0:55:30An electron somewhere out there
0:55:30 > 0:55:33radiated a photon
0:55:33 > 0:55:35which hit an electron
0:55:35 > 0:55:38in your eye.
0:55:38 > 0:55:41Alien photons crossing light years of space,
0:55:41 > 0:55:45entering your retina and carrying stories of distant worlds.
0:55:52 > 0:55:56In the constellation of Pegasus, around a sun-like star,
0:55:56 > 0:55:58there's a planet called Osiris.
0:55:58 > 0:56:03It's way closer to its parent star than Mercury is to our sun,
0:56:03 > 0:56:07but it's a massive gas giant, a hot Jupiter.
0:56:07 > 0:56:10And by measuring the light that shines through its atmosphere,
0:56:10 > 0:56:16we found that the atmosphere is full of hydrogen and methane and water.
0:56:19 > 0:56:23And just 24 light years away in the constellation of Scorpius,
0:56:23 > 0:56:28there's a small, rocky world, one of the most earthlike worlds we know.
0:56:28 > 0:56:31It orbits around a red dwarf star,
0:56:31 > 0:56:35which is one of three stars in the system.
0:56:35 > 0:56:38A planet with three suns in the sky.
0:56:42 > 0:56:44In the last 20 years,
0:56:44 > 0:56:48we've found hundreds of planets around distant suns.
0:56:48 > 0:56:50We've began to analyse their atmospheres,
0:56:50 > 0:56:53and search for life-giving water.
0:57:01 > 0:57:04Science is about gathering data,
0:57:04 > 0:57:06information about the natural world,
0:57:06 > 0:57:07and trying to understand it.
0:57:07 > 0:57:10And put like that, it seems a bit dry.
0:57:10 > 0:57:15But when that information carries stories of alien worlds
0:57:15 > 0:57:18around distant suns hundreds of light years away,
0:57:18 > 0:57:20that's a whole different thing.
0:57:20 > 0:57:24Astronomy turns data into dreams -
0:57:24 > 0:57:27dreams of worlds of ice and snow,
0:57:27 > 0:57:30dreams of world with hemispheres
0:57:30 > 0:57:33in perpetual day and permanent night.
0:57:33 > 0:57:36Dreams of world with moons and moonbows,
0:57:36 > 0:57:39and perhaps, just perhaps,
0:57:39 > 0:57:42alien astronomers observing the light
0:57:42 > 0:57:45passing through the atmosphere of our blue world,
0:57:45 > 0:57:47and dreaming of us.